Environmental Engineering Reference
In-Depth Information
plantings. Yield data from small plots are higher than larger ones because of an edge effect bias
(Zavitkovski 1981). The yields across the continent also vary with factors such as species, clone,
site conditions, region, cultural methods, spacing, harvest strategy, pests and diseases (Hansen et al.
1992; Dickmann 2006). Poplars are particularly sensitive to competition for available light, nutri-
ents and water. Therefore, weed control and nutrition were determined to be essential for maintain-
ing survival, growth and yield of planted poplars (Aird 1962; McKnight and Biesterfeldt 1968;
van Oosten 2006; Isebrands 2007). For example, Kennedy (1975) found weed control essential for
eastern cottonwood plantings. Czapowskyj and Stafford (1993) found that mowed and fertilized
poplar plantations in Maine grew 4 times as fast as the untreated control areas. And Coleman et al.
(2006) found that post-establishment fertilized operational poplar plantations grew 3.5 times as fast
as unfertilized stands and that fertilizer increased biomass by 40% in the third year.
The early plantations of poplars in North America were planted largely with improved native
species (Smith and Blom 1966; McKnight and Biesterfeldt 1968; Heilman et al. 1972). Black cot-
tonwood plantations in the Pacific Northwest grew from 2 to 7 mt/ha per year at close spacing and
from 9 to 11 mt/ha per year after coppicing (i.e., resprouting) (Smith and DeBell 1973). Mean annual
biomass accumulation of cottonwood plantations in the lower Mississippi Valley averaged approxi-
mately 10-13 mt/ha per year at 3 × 3 m spacing with the maximum rate occurring at age 5 without
thinning (Krinard and Johnson 1975; Switzer et al. 1976). Francis and Baker (1981) reported annual
biomass production of a commercial plantation of a selected cottonwood clone at 3 × 3 m spacing
to be 7.6 mt/ha per year in 4 years.
With the advent of the short rotation forestry concept, researchers were anxious to test all of the
aforementioned yield factors; many started with small plots. Moreover, forest geneticists quickly
designed genetic trials throughout North America to test their new and sometimes old poplar clones
(Schreiner 1959; Garrett 1976; Randall 1976; Stettler et al. 1992). In some cases, the native spe-
cies outgrew the ill-adapted new clones (Blow 1948; Smith 1968; Maisenhleder 1970). There was
an early trend toward very close spacing trials with repeated coppicing (Blake 1983; Ferm and
Kauppi 1990). Spacing was determined as an important factor for biomass yields, and it had to be
coupled properly with rotation age to maximize yield. Early results from Wisconsin in the midwest-
ern United States with a Canadian hybrid poplar by Ek and Dawson (1976) planted at 0.22, 0.3 and
0.6 m
2
spacing were 11.3, 12.6 and 13.8 mt/ha per year, respectively after 4 years. After 10 years the
highest biomass production from a clone and spacing study in Wisconsin was 10.4 mt/ha per year at
a 2.4 m
2
spacing (Zavitkovski 1983). These Wisconsin studies were with poplar clones from the NE
poplars and from Canadian cooperators Cram (1960) and Zsuffa (1975). Strong and Hansen (1993),
in an overview of 16 years of the Wisconsin studies, stated the maximum mean annual biomass
production was 12.8 mt/ha per year by an NE clone planted at 1 × 1 m spacing. They concluded that
biomass yield was independent of spacing except that time to maximum biomass (i.e., rotation) and
tree size will vary according to spacing.
Block demonstration plantings throughout the midwestern United States of the NE clones planted
at 2.4 × 2.4 m spacing yielded from 4.7 to 9.5 mt/ha per year in 8 yrs. The mean annual biomass
production peaked at year 10 at 7 mt/ha per year and the “best” sites averaged 9.2 mt/ha per year
(Netzer et al. 2002). Berguson (2008) recently presented results of yield tests for commercially
managed poplar plantations in the Midwest; yields of current clones produce from 7.8 to 12.3 mt/
ha per year.
L. Zsuffa worked in Ontario on poplar breeding and short-rotation forestry throughout his career
(Richardson et al. 2007). The “best” poplar clones from his program yielded between 5 and 19 mt/
ha per year (Zsuffa 1975; Zsuffa et al. 1977; Anderson 1979). The Ontario Ministry of Forestry
growers guide for hybrid poplar presents clonal descriptions and biomass tables for Canadian clones
(Boysen and Strobl 1991). At the same time as the Ontario work, G. Vallee and co-workers estab-
lished a poplar breeding program and conducted biomass trials in Quebec. Results from that breed-
ing program are presented in Dickmann et al. (2001). Labreque and Teodoreseu (2005) reported
hybrid poplar yields of 17-18 mt/ha per year at 0.3 × 1.7 m spacing in a coppice study in Quebec.
